You'll become part of the founding team; together we'll architect and realize Sproost's vision.
Suggests that a considerable amount of equity compensation could be involved.
But yeah, when they said "first hire", that was my thought as well -- they basically want a technical cofounder / chief architect sort of person.
Hired people work for salary. The kind of people who qualify either command high salaries or shares.
Other than that it sounds like a nice job for a 20-year old hacker wanting to get some experience and buff up that resume in the process.